Conventionally, a cationic polymer flocculent is used as a retention aid in paper making and for dewatering sewage sludge and the like. In recent years, the amount of generated sludge is increasing, and the properties of the sludge are becoming worse. However, with conventional cationic polymer flocculants, the amount of treatable sludge is limited, and the treatment conditions are not satisfactory in terms of the water content of a dewatered cake, the SS recovery rate, the removability of a cake from filter cloth, and the like. Therefore, there is a strong demand to improve sludge dewatering agents. To improve the drawbacks of conventional cationic polymer flocculants, various amphoteric polymer flocculants have been proposed. However, such amphoteric polymer flocculants are not fully satisfactory.
Moreover, crosslinking ionic polymer flocculants have been disclosed for the purpose of reducing the water content of a dewatered cake and of improving the removability from filter cloth (see, for example, Japanese Patent Application Laid-Open No. Hei 02-219887 (European Patent Application Laid-Open No. 374458 A2) and Japanese Patent Application Laid-Open No. Sho 61-293510 (U.S. Pat. No. 4,720,346).
As described above, crosslinking water-soluble polymers have various characteristics and functions. However, disadvantageously, when such a crosslinking water-soluble polymer is used in a sludge dewatering agent, a retention aid in paper making, or the like, the amount required to be added is greater than those of linear polymers. This may be because the extension of crosslinking water-soluble polymer molecules in an aqueous solution is smaller relative to that of linear polymers. To improve such a drawback of the crosslinking water-soluble polymers, amphoteric polymer dewatering agents have been disclosed which contain quaternary ammonium salt groups and are produced by copolymerizing a methacrylate-based monomer, an acrylate-based monomer, and an anionic monomer in a certain ratio (Japanese Patent Application Laid-Open Nos. Hei 7-256299 and Hei 7-256300). However, these also do not fully satisfy the demand.
When a crosslinking water-soluble polymer is used as a sludge dewatering agent or a retention aid, such advantages can be obtained that the water content of a dewatered cake can be reduced and good formation can be easily maintained. However, the amount of the polymer added must be increased to achieve the advantages, and this results in an increase in cost.
To solve this problem, a technique has been disclosed in which a crosslinking water-soluble ionic polymer and a linear water-soluble ionic polymer are compounded to exploit the advantages of both the polymers (Japanese Patent Application Laid-Open No. 2004-57837).
However, this technique does not describe the concept that the compounding ratio of the crosslinking water-soluble ionic polymer to the linear water-soluble ionic polymer is changed according to the size distribution of sludge particles.
It is an object of the present invention to develop a flocculant composition applicable to various types of sludge and a method for manufacturing the same. Specifically, the concept of exploiting the advantages of a crosslinking water-soluble ionic polymer and a linear water-soluble polymer is further extended, i.e., the compounding ratio of the crosslinking water-soluble polymer to the linear water-soluble polymer is adjusted according to the size distribution of sludge particles.